In various situations, it can be desirable to prevent articles from being removed from an area or to detect the movement of articles within an area. For example, it may be desirable to prevent the removal of articles from a shop that have not been paid for i.e. to prevent shoplifting.
One approach is electronic article surveillance (EAS). Methods of EAS rely on specialised tags attached to the articles. Detectors, often placed at the exit of the area, are able to detect the tag and thus the article to which the tag is attached. An alarm may be activated if the detector detects a tag leaving the area. The tags may be detached from the article or deactivated so that the article can be removed from the area without the alarm being activated.
Typically, the tag will include a suitable antenna that generates a field able to be detected by a corresponding antenna or reader in the detector. Alternatively, the tag may detectably interfere with a field being generated by the detector itself. Some known approaches rely on, for example, magnetic, acousto-magnetic or radio frequency fields.
A downside of this approach to EAS is that it can be difficult to determine the position of the tag (and thus the article) with a suitable degree of precision. This makes it difficult to determine if and when the article is being removed from the area.
In some approaches, the detector may be able to scan for tags in multiple directions, and from the angle of the signal determine the approximate position of a tag. For this measurement, it may be necessary to presume the height of the tag off the floor, which may in fact not be accurate. The strength of the signal may be used to approximate further the position of the tag. However, the strength of the signal is dependent on the angle of the tag antenna relative to the detector antenna. For at least these reasons, these approaches may not be sufficiently precise or accurate at determining the position of a tag.
Another approach is to provide gate-like detectors at the exit of an area through which people have to pass. Though these detectors give increased precision, they can be unsightly and are limited to well defined exits (such as, for example, a doorway).
It is therefore desirable to provide a method and device for determining the position of a tag with improved accuracy and/or precision. There is also a need for providing a method and device for determining the position of a tag that is not dependent on obtrusive gate-like detectors.